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CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (Unaudited) - USD ($) $ in Millions | Total | Common shares and paid-in surplus | Retained earnings | AOCI, after tax | Total | Non-controlling interest | Total Equity | Foreign Currency Gain (Loss) Common shares and paid-in surplus | Foreign Currency Gain (Loss) Retained earnings | Foreign Currency Gain (Loss) AOCI, after tax | Foreign Currency Gain (Loss) Total | Foreign Currency Gain (Loss) Non-controlling interest | Foreign Currency Gain (Loss) Total Equity | Pension liability and other accumulated comprehensive items AOCI, after tax | Pension liability and other accumulated comprehensive items Total | Pension liability and other accumulated comprehensive items Total Equity | OneBeacon Common shares and paid-in surplus | OneBeacon Retained earnings | OneBeacon AOCI, after tax | OneBeacon Total | OneBeacon Non-controlling interest | OneBeacon Total Equity | Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Common shares and paid-in surplus | Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Retained earnings | Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Non-controlling interest | Sale of Sirius Group Common shares and paid-in surplus | Sale of Sirius Group Retained earnings | Sale of Sirius Group AOCI, after tax | Sale of Sirius Group Total | Sale of Sirius Group Non-controlling interest | Sale of Sirius Group Total Equity | Star & Shield Insurance Exchange Common shares and paid-in surplus | Star & Shield Insurance Exchange Retained earnings | Star & Shield Insurance Exchange AOCI, after tax | Star & Shield Insurance Exchange Total | Star & Shield Insurance Exchange Non-controlling interest | Star & Shield Insurance Exchange Total Equity
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Beginning Balances at Dec. 31, 2016 |  | $ 810.7 | $ 2,776.6 | $ (4.6) | $ 3,582.7 | $ 133.3 | $ 3,716.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net (loss) income | $ 581.1 | 0.0 | 604.7 | 0.0 | 604.7 | (23.6) | 581.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation and other |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.4 | $ 0.4 | $ 0.1 | $ 0.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2.9 | $ 2.9 | $ 2.9 |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 |  |  |  |  |  |  |  |  |  |  |  |
Total comprehensive income (loss) | 608.1 | 0.0 | 604.7 | 3.3 | 608.0 | (23.5) | 584.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  | 0.0 | (4.6) | 0.0 | (4.6) | 0.0 | (4.6) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends to non-controlling interests |  | 0.0 | 0.0 | 0.0 | 0.0 | (17.7) | (17.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares |  | (147.9) | (575.9) | 0.0 | (723.8) | (5.2) | (729.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 1.7 | 0.0 | 0.0 | 1.7 | 0.0 | 1.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dilution from equity-based units of subsidiary |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (4.1) | $ 0.0 | $ 0.0 | $ (4.1) | $ 4.7 | $ 0.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital contributions from BAM members, net of tax | (25.7) | 0.0 | 0.0 | 0.0 | 0.0 | 18.7 | 18.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share awards |  | 10.6 | 0.0 | 0.0 | 10.6 | 0.8 | 11.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deconsolidation of non-controlling interests associated with the sale of Star & Shield |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ (238.3) | $ (238.3) | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ (4.4) | $ (4.4)
Net contributions from non-controlling interests |  | (1.7) | 0.0 | 0.0 | (1.7) | 0.0 | (1.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending Balances at Sep. 30, 2017 |  | 669.3 | 2,800.8 | (1.3) | 3,468.8 | (131.6) | 3,337.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning Balances at Dec. 31, 2017 | 3,360.8 | 670.6 | 2,823.2 | (1.3) | 3,492.5 | (131.7) | 3,360.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net (loss) income | (50.9) | 0.0 | (3.7) | 0.0 | (3.7) | (47.2) | (50.9) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation and other |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ (1.5) | $ (1.5) | $ 0.0 | $ (1.5) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total comprehensive income (loss) | (5.4) | 0.0 | (3.7) | (1.5) | (5.2) | (47.2) | (52.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  | 0.0 | (3.8) | 0.0 | (3.8) | 0.0 | (3.8) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends to non-controlling interests |  | 0.0 | 0.0 | 0.0 | 0.0 | (6.0) | (6.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares |  | (104.6) | (408.4) | 0.0 | (513.0) | 0.0 | (513.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 2.0 | 0.0 | 0.0 | 2.0 | 0.0 | 2.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of equity-based units of subsidiary |  | 4.5 | 0.0 | 0.0 | 4.5 | 2.3 | 6.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dilution from equity-based units of subsidiary |  | (1.1) | 0.0 | 0.0 | (1.1) | 1.1 | 0.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital contributions from BAM members, net of tax | (26.9) | 0.0 | 0.0 | 0.0 | 0.0 | 22.9 | 22.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share awards |  | 9.7 | 0.0 | 0.0 | 9.7 | 0.0 | 9.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest from acquisition |  | 0.0 | 0.0 | 0.0 | 0.0 | 14.2 | 14.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition from non-controlling interest |  | (1.6) | 0.0 | 0.0 | (1.6) | 0.0 | (1.6) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending Balances at Sep. 30, 2018 | $ 2,839.6 | $ 579.5 | $ 2,407.3 | $ (2.8) | $ 2,984.0 | $ (144.4) | $ 2,839.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4568740-111683 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 55 -Paragraph 4I -URI http://asc.fasb.org/extlink&oid=84234705&loc=SL4590271-111686 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4568447-111683 + Details
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